1. Field of the Invention
The present invention relates generally to active matrix electroluminescent (AMEL) displays, and more particularly relates to an AMEL pixel driving circuit and quasi-analog method of operating same for improved gray scale and color operation.
2. Description of the Related Art
Thin film electroluminescent (EL) displays are well known in the art and are typically used for flat screen displays in a variety of applications. A typical display includes a plurality of pixels arranged in an array of rows and columns. Each pixel comprises an electroluminescent phosphorus active layer interposed between a pair of insulators and a pair of electrodes. A typical embodiment is a 640.times.480 pixel array placed in a 0.75 inch square area for use in heads-up displays and the like.
The pixels in a conventional AMEL display are either active (illuminated) or inactive (dark) at any given time. To generate shades of gray, typical AMEL displays are operated in a time multiplexed mode. Referring to FIG. 1, prior art AMEL displays generally include a driver circuit at each pixel comprising a first transistor 100 having a gate connected to a select line 110, a source connected to a data line 108 and a drain connected to a gate of a second transistor 102 and through a first capacitor 104 to an AC ground potential. A source of the second transistor 102 is connected to a ground terminal, a drain is connected to one electrode of an EL cell 106. A second electrode EL cell 106 is connected to a high voltage alternating current source (AC Source) 112 for excitation of the phosphor in the EL cell 106.
A conventional AMEL pixel is operated in consecutive time frames. During a first portion of a time frame (load cycle) a digital signal is consecutively applied to all data lines 108 in the display. For those pixels which will be active, the corresponding select lines 110 are strobed consecutively by row. This turns on transistor 100 thereby allowing charge from data line 108 to accumulate on capacitor 104 and on the gate of transistor 102, thereby turning transistor 102 on. The charge is stored in capacitor 104 when the select line 110 returns to an inactive state. At the completion of the load cycle, the second transistor 102 associated with each activated pixel is on. During a second portion of the frame (illumination cycle), the AC source 112 is applied to all pixels in the display. Current flows from the AC source 112 through the EL cell 106 and the transistor 102 to ground in each activated pixel. This produces an electroluminescent light output from each activated EL cell 106. For each load/illuminate frame, each pixel is either on or off. Therefore, gray scale operation is only achieved by time multiplexing the pixel over a sequence of multiple load/illumination frames and allowing the eye to integrate the light pulses into a gray image.
Efforts have been made in the prior art to improve the gray scale operation of AMEL display devices. For example, U.S. Pat. No. 5,302,966 to Stewart discloses an AMEL display that employs an analog driving technique for each pixel. FIG. 2 illustrates one embodiment disclosed in the Stewart patent. This circuit includes a first transistor 200 having a gate terminal coupled to a select line 210, a drain terminal connected to a data line 208 and source terminal connected to the gate terminal of a second transistor 202. The source terminal of the first transistor 200 is also connected to capacitor 204 which has a second terminal coupled to circuit ground. The second transistor 202 has a source terminal connected to the data line 208 and a drain terminal connected to an EL cell 206. The EL cell 206 is further connected to an AC source 212.
As with the driver circuit of FIG. 1, the circuit of FIG. 2 operates in consecutive frames having load/illuminate cycles. However, during the load cycle, an analog voltage is applied to the data line 208 and this analog voltage is stored in capacitor 204 when an active pixel receives a row select strobe. During the subsequent illumination cycle, the data line receives an analog ramp signal. So long as the ramp signal has a voltage potential less than the voltage stored in capacitor 204 (minus a threshold), transistor 202 conducts, thereby illuminating EL cell 206. When the ramp voltage on the data line exceeds the stored voltage in capacitor 204 minus the threshold, transistor 202 turns off and current no longer flows through EL cell 206. Therefore, El cell 206 is only operating during a portion of each illumination cycle, thus providing a variable illumination period during each frame which provides gray scale operation.
To achieve 64 levels of gray scale using the method of the Stewart patent, all of the transistors in the AMEL driver circuit of FIG. 2 must consistently respond to a small change in voltage equal to (V.sub.DD -V.sub.DMOS)/64, where V.sub.DMOS is a threshold voltage where the transistors are active. For a threshold voltage of 2 volts and V.sub.DD of 5 volts, the resolution of each voltage step is about 47 millivolts. However, current manufacturing technology and the errors related to high voltage coupling components do not provide sufficient uniformity across the AMEL display substrate to support this requirement. Therefore, the step size must be increased either by using non-conventional power supply voltages or by reducing the number of gray scale steps of the display.
The operation of the AMEL display in accordance with the Stewart patent also supplies a highly non-uniform current versus time profile. When only the least significant bit is active, the EL cell 206 will be active for only 1 out of 32 pulses. When the most significant bit (MSB) is active, the EL cell will be active for all 32 pulses. This broad range of duty cycle (32:1) makes power recovery techniques impractical, thereby reducing the overall operating efficiency of the display.
Therefore, there remains a need for an improved AMEL display. Such a display shall feature a more uniform activation profile and also feature increased step size for a given resolution of gray scale operation. Such circuits and methods will also be applicable to color displays as well as monochrome displays.